The insertion loss flatness of coupled resonator microwave bandpass filters is affected by an undesired slope. This undesired slope is due to the presence of dissipative losses which occur at the couplings between adjacent resonators. It can lead to a non compliance of the targeted filter specifications, in particular at low frequencies.
Therefore, there is a need for a method for equalizing the distortion due to this phenomenon in order to minimize the slope of filters in their bandpass.
It is known from the prior art, in particular in references [1],[2] and [3], methods for pre-distortion of a filter transfer function which are intended to compensate for the rounding that occurs at both edges of the filter pass band due to the dissipation in the filter cavities.
However these techniques only consider the unloaded quality factor of resonators, in other words the quality factor of isolated resonators, but do not take into account the effect of the finite quality factor of pure reactive elements, i.e. inductive or capacitive couplings between adjacent resonators due to losses in these elements.
It is also know from reference [5] the use of the quality factor of couplings between resonators to calculate the normalized impedance of a resonator filter. However, reference [5] does not teach that the quality factor of couplings can be used to pre-distort the transfer function of a filter in order to equalize and correct the insertion loss variation of the filter.
Therefore, there is a need for a method for equalizing insertion loss variations in a filter transfer function and compensating the effect of losses in couplings between adjacent resonators used for implementing the filter.
A goal of the invention is to provide a deterministic method for equalizing insertion loss variations in a filter transfer function. The filter electrical parameters are modified in order to equalize the insertion loss slope due to dissipative couplings, while maintaining selectivity and group delay characteristics.